1. Field of the Invention
The present invention is generally related to a measurement apparatus for the thickness of a cake growing in a filtration system, and more particularly to an apparatus for in-situ measuring the thickness of a grown cake by utilizing a photo sensing device together with a driving device to incorporate in a filtration system.
2. Description of the Prior Art
In a filtration process, as the solid content in a slurry has a diameter larger than the pore diameter of a filter medium, after the filtration cycle proceeds several times, a cake gradually forms on the surface of the filter medium and the thickness of the cake increases with increase in time. As the thickness of the cake is thicker, the obstruction in filtering the slurry becomes larger and thus the filtration rate becomes slower. Thus, the cake on the filter medium should be removed. However, usually the cake is hard to be completely removed and leaves a portion on the filter medium. If the above filter medium is continued to be used in the filtration process, the difference in the thickness distribution of the formed cake becomes larger and larger to cause the flow of the slurry unstable while the physical properties of the cake, such as the viscosity and the porosity of the cake, also become more uneven. Therefore, if the thickness distribution of the cake can be measured in time, the physical properties of the cake can be realized. That is, a reverse cleaning process can be taken place while the thickness of the cake is measured to reach a certain value so as to increase the filtration efficiency. In the fields like exhaust gas filtration, slurry filtration, or precision ceramics, the method for measuring the thickness distribution of the cake is a very important issue.
Currently, the technology in measuring the thickness of a cake is roughly divided into two categories: (1) a destructive measurement method: taking out the cake from the filter medium, shaping the cake to prepare for being easily measured, that is, a method destroying the original structure of the cake and incapable of in-situ measuring the thickness of the cake during the filtration process; and (2) a non-destructive measurement method: directly measuring the thickness of the cake formed during the filtration process without interfering the formation of the cake and destroying the structure of the cake, that is, a method measuring the real thickness of the cake during the filtration process.
The destructive measurement comprises methods like a drying and weighting method, a frozen section method, a cake reconstructing method, and so forth. Some report disclosed that a few points on the cake are chosen to analyze the protein content in the cake formed by cells to estimate the number of cells per unit volume of the cake and the shape of the cell and the porosity are predetermined so as to calculate the thickness of the cake. A microscope is used to scan yeast and cells to verify this measurement method but the report did not show the comparison result. This method is very complicate and very easy to create errors (Riesmeier, B., K. H. Kroner, and M. R. Kula, Studies on secondary layer formation and its characterization during cross-flow filtration of microbial cells, J. Membr. Sci., Vol. 34, pp. 245, 1987). The report about frozen section is to freeze the cake and then observe the thickness and the porosity of the cake by a microscope (Schmidt, E. and F. Loffeler, Preparation of dust cakes for microscopic examination, Powder Technol., Vol. 60, pp. 173, 1990). Additionally, a destructive method for measuring the thickness and the porosity of the cake is disclosed where the solid content of the cake formed on the tubular filter medium is taken out, the surface filtration method (filtration under constant pressure) under the same operating conditions is used to reconstruct the cake, and the thickness is measured as the thickness of the cake in the tubular filter medium (Vyas, H. K., A. J. Mawson, R. J. Bennett, A. D. Marshall, A new method for estimating cake height and porosity during cross flow filtration of particulate suspensions, J. Membr. Sci. Vol. 176, pp. 113-119, 2000).
Various non-destructive measurement methods have also been developed currently. Mostly light or sound waves are used and then the physical properties of the cake are determined by the intensity of their reflective waves. For example, U.S. Pat. No. 3,748,263 in 1973 disclosed a method of measuring the thickness of a cake on a filter element of a settling tank filter where the filter pads are vertically arranged and the slurry fed to the filter is divided into the cake and the filtrate. The cake is formed on the surface of the filter pad and the filtrate flows through a branched off stream and is collected by the collecting tank. When the cake has grown to its intended maximum thickness, it closes the passage means through the filter body while a filter layer settles on the filter body and filters the slurry of the branched off stream, the thus obtained filtrate being visible through a sight glass or the like and thus indicating that the filter cakes in the settling tank filter have reached their admissible maximum thickness.
U.S. Pat. No. 3,815,745 in 1974 disclosed a device for evaluating the quality of cake in intermittent filters. The device measures the thickness of a cake, washing degree, and drying degree by introducing a filtrate, washing liquid, and dried gas to the filter through a manifold where a sensing element is provided between the manifold and the inlet of the filter and the other sensing element is provided in the filter so that the signal difference of the two sensing elements are used to measure the thickness of the cake. The similar method is also applied to measure the washing degree and drying degree. Thus, the thickness of a cake, the washing degree, and the drying degree can be controlled.
Moreover, U.S. Pat. No. 4,548,080 in 1985 disclosed an apparatus to simulate fluid dynamics during drilling a well. The apparatus comprises a probing element having an ultrasonic device to generate and receive ultrasonic waves. The pole-like probing element is around and spaced from the porous cylindrical sleeve by a distance. The cake from the slurry mud is formed on the inside of the porous cylindrical sleeve. The ultrasonic device generates an ultrasound signal radially outwardly from the probing element and then the signal is reflected back by the cake on the porous cylindrical sleeve. The length of time taken for the echo to be picked up by the receiver can be used to determine the thickness of the cake. Thus, the flow rate flowing through the cake can be maintained.
It is reported that a method of measuring the height distribution on the surface of a filter cake by using a fringe-type light projector to moves light beams on the filter cake, incorporating with a CCD digital camera to record the angle difference of the intersection point between the filter medium and the surface of the filter cake with respect to the light projector, and calculating and showing its 3D images (Dittler, A., B. Gutmann, R. Lichtenberger, H. Weber and G. Kasper, Optical in situ measurement of dust cake thickness distributions on rigid filter media for gas cleaning, Powder Technology, Vol. 99, pp, 177-184, 1998).
Furthermore, a measurement method is disclosed by Hutchins D. A. and H. D. Mair, Ultrasonic monitoring of slip, cast ceramics, J. Materials science Letters, Vol. 8, pp. 1185, 1989. The time between the ultrasound generated and reflected is used to measure the thickness of the cake formed during the precision ceramic process. But, the method interferes the formation of the cake or destroys the structure of the cake.
The destructive measurement method cannot in-situ acquire the thickness of the cake during the filtration process and thus cannot immediately make adjustment. Besides, the original structure of the cake is changed during drying, freezing, reconstructing the cake. On the other hand, the above mentioned non-destructive measurement method for measuring the filter cake thickness has problems, such as requiring expensive equipments, complicated conversion after measurement, time consuming, generating experimental or calculation errors, and destroying the formation of the cake or the structure of the cake due to the radiation on the cake.